Further, there are various types of modulation methods used in transmitters of microwave digital fix radio communication devices. Some devices are configured to use some modulation methods by switching them. In this case, each modulation method has different standards of a spectrum mask for a transmitting output, and in general, low multiple values (QPSK, BPSK or the like) have a large acceptable value of the non-linear distortion and high multiple values (128 QAM, 256 QAM or the like) have a small acceptable value of it. That is, in the low multiple values, a large transmitting power can be obtained while allowing the distortion. But in the high multiple values, the large transmitting power cannot be obtained because of an influence of the distortion.
An adjustment of the output power is often performed by connecting plural amplifiers with multiple stages and changing an operating point of each amplifier in order to change a maximum value of the transmitting power depending on the spectrum masks. When a certain degree of distortion can be allowed in the low multiple values, each operating point of the former and later stage amplifiers is set higher and a high output power can be obtained. When much distortion amount cannot be allowed in the high multiple values, one possible method is such that a gain of the former stage amplifier is set lower and the later stage amplifier amplifies the output power to a desired value.
As described above, in the transmitter of the radio communication device, when a desired output power cannot be obtained by using one amplifier, a technique is performed in which the amplifiers are connected with multiple stages to obtain the desired output power. Further, a non-linearly correction circuit (hereinafter, referred to as a linearizer) is used to restrain a frequency component out of a band frequency that is caused by the non-linear distortion in the amplifier. As the linearizer, for example, a feed forward type linearizer is used where a feedback from the transmitting output is not used.
Documents disclosing a technique related to the amplifier and the linearizer as described above include, for example, Patent document 1, Patent Document 2.
Citation List
Patent Literature
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-069731
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-239190